Sensirion SDP sensors measure pressure but even a flow

Calibrated differential pressure sensors will find usage in industry and also in every smart home.

This is an archive article published 15.11.2013. Some information may no longer be up to date and in line with the current state. Please contact us in case of interest.

Ventilation, air conditioning (HVAC), or any other applications, where a pressure or a flow of air (gas) is essential, can be precisely regulated only if we know a real pressure/ flow conditions. Monitoring of fans functionality, status of filters, …, even for this purpose are very suitable differential pressure sensors from Swiss company Sensirion with an excellent accuracy and long-term stability.

As SDP sensors are differential, they have 2 nozzles. The base of these sensors is a miniature heating element and two temperature sensors. Difference of pressures will cause a small air flow through a component and it will cause a temperature difference in these built-in sensors. Temperature difference is directly depending on the amount (weight) of air flowing through a sensor and it naturally depends just on a pressure difference. A great advantage against membrane pressure sensors is a long-term high accuracy even at measuring of small pressure and maintaining of accuracy of a “zero point”.

As an air specific weight depends on temperature, measurement based only on a temperature difference of two internal sensors wouldn´t be accurate. That´s why SDP sensors have a built-in temperature compensation and linearization, thus an output of a sensor is directly a value usable for further processing. Resolution of sensors is very high and is user selectable in a range of 9-16 bits. Resulting binary value read-out through I2C interface can be divided by a constant shown in a tab. 2.1 on page 3 of the SDP6xxv1.7 datasheet and we´ll get a resulting value in desired units (Pa, bar, inch).

Universal and price-attractive series is the SDP600 (6xx) series, in which we´ll find types optimized for measurement of pressure (SDP 600/610), mass flow (SDP601/611) and types with a minimal power consumption suitable even for battery-operated devices (SDP606/616). All these types are able to measure “bidirectionally” i.e. positive and even negative values. For applications, where we know, that a pressure difference will be always positive (flow by only one direction), it´s possible to use more price-affordable types SDP 500/510 (in fact even these types measure bidirectionally, but their accuracy is not guaranteed in this range). For the most of applications no additional compensation is necessary, only it can be necessary to multiply a resulting value by a constant for an altitude compensation shown in a table 5.4 on page 6 in the datasheet.

How can be this small sensor used to measure flow of a big amount of air?

Relatively simply. All that is necessary is to make a small orifice in a pipe and this way to create a “resistance” as described in the SDPxxx_Bypass_Configuration application note. In an electronics analogy the differential pressure measurement is similar to measurement of voltage on a resistor, which depends on a current flowing through a resistor. SDP sensors can be used even for measurement of a pressure difference in a given pipe against a surrounding environment, in such a way, that one nozzle of sensor will be left “unconnected”. Naturally, such a configuration can only be used if a small leakage of a measured “gas” (air) is acceptable.

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